The present invention relates to a carburizing steel composition, to parts formed from said steel, and to a process for producing parts formed from said steel.
Carburizing is a thermochemical surface treatment which generally produces parts combining good core ductility with a xe2x80x9ccase-hardenedxe2x80x9d carburized surface that is hard and resistant to wear.
Many applications require a steel with a good resistance to softening at working temperatures. Examples that can be cited are gear wheels, bearings and transmission shafts for helicopters or for vehicles for motor racing, gear wheels, camshafts and other parts used in engine distribution systems, fuel injectors and compressors.
The following particular carburizing steels are routinely used for such applications: 17CrNiMo6, 16NiCr6, 14NiCr12, 10NiCrMo13, 16NiCrMo13 or 17NiCrMo17. Such steels can be used up to working temperatures of close to 130xc2x0 C., but the carburized layer has neither a resistance to softening nor an elevated temperature hardness sufficient for working temperatures exceeding 190xc2x0 C.
U.S. Pat. No. 3,713,905, granted to T. V. Philip and R. L. Vedder on Jan. 30th, 1973, describes the properties obtained for a steel with the following chemical composition as a percentage by weight:
0.07%-0.8% of C;
at most 1% of Mn;
0.5%-2% of Si;
0.5%-1.5% of Cr;
2%-5% of Ni;
0.65%-4% of Cu;
0.25%-1.5% of Mo;
at most 0.5% of V;
the complement being iron.
The tensile strength and the impact strength obtained with that steel are compatible with the envisaged applications, but the tempering properties and the elevated temperature hardness of the carburized layer are insufficient for the applications cited above and for working temperatures of up to 220xc2x0 C.
U.S. Pat. No. 4,157,258, granted to T. V. Philip and R. L. Vedder on Jun. 5th 1979, describes a steel with the following chemical composition as a percentage by weight:
0.06%-0.16% of C;
0.2%-0.7% of Mn;
0.5%-1.5% of Si;
0.5%-1.5% of Cr;
1.5%-3% of Ni;
1%-4% of Cu;
2.5%-4% of Mo;
xe2x89xa60.4% of V;
xe2x89xa60.05% of P;
xe2x89xa60.05% of S;
xe2x89xa60.03% of N;
xe2x89xa60.25% of Al;
xe2x89xa60.25% of Nb;
xe2x89xa60.25% of Ti;
xe2x89xa60.25% of Zr;
xe2x89xa60.25% of Ca
the complement being iron.
The compromise between tensile strength and impact strength for that steel is good. The carburized layer allows a tempering temperature of up to about 260xc2x0 C. The maximum working temperature is about 230xc2x0 C.
However, none of the prior art carburizing steel compositions can allow a tempering temperature for the carburized layer of up to 350xc2x0 C. to be used, nor do they provide good elevated temperature hardness for working temperatures of up to 280xc2x0 C. while preserving satisfactory core characteristics.
There is currently a need for such steels in a number of fields. As an example, regarding the manufacture of gear parts for helicopters, regulations require that a helicopter must be capable of functioning for thirty minutes after losing oil from its transmission following an incident. That requirement assumes that the materials used to manufacture the gears have been tempered at a minimum temperature of about 280xc2x0 C.
In the field of engines, designers tend to increase the working temperature of engine parts and its connected equipment such as gearboxes, in order to increase yields and/or to simplify heat extraction circuits. Depending on the location of the parts in this equipment, working temperatures can reach 280xc2x0 C., imposing a minimum tempering temperature of 330xc2x0 C. to guarantee that properties are stable during use.
The present invention aims to provide a carburizing steel composition that has all of the characteristics mentioned above.
In a first aspect, the invention provides a carburizing steel composition comprising, by weight:
0.06% to 0.18% of C;
0.5% to 1.5% of Si;
0.2% to 1.5% of Cr;
1% to 3.5% of Ni;
1.1% to 3.5% of Mo;
and, if appropriate:
at most 1.6% of Mn; and/or
at most 0.4% of V; and/or
at most 2% of Cu; and/or
at most 4% of Co;
the complement being constituted by iron and residual impurities;
the weight contents of Ni, Mn, Cu, Co, Cr, Mo and V in said composition, expressed by weight, satisfying the following relationships:
2.5 xe2x89xa6Ni+Mn+1.5Cu+0.5Coxe2x89xa65xe2x80x83xe2x80x83(1)
xe2x80x832.4xe2x89xa6Cr+Mo+Vxe2x89xa63.7xe2x80x83xe2x80x83(2).
Preferably, the sulfur content is limited to 0.010% and the phosphorous content is limited to 0.020% by weight, for applications in the upper part of the range, but higher contents are acceptable for other applications, provided that they do not cause a reduction in the ductility, toughness and fatigue strength properties of the steel.
The amount of elements such as aluminum, cerium, titanium, zirconium, calcium or niobium, which act either to deoxidize or to refine grain size, is preferably limited to 0.1% by weight each.
Regarding the principal elements of the composition, in general it has been shown that low carbon, silicon, molybdenum, chromium, and vanadium contents, and high manganese, nickel, cobalt, and copper contents can improve the ductility and toughness of the steel.
In contrast, high carbon, silicon, molybdenum, chromium, and vanadium contents and low manganese, nickel, cobalt, and copper contents can improve the tempering strength of the steel.
The essential role of carbon is to contribute to producing hardness, tensile strength, and hardenability. For carbon contents of less than 0.06% by weight, the hardness and tensile strength obtained in the core of carburized and treated parts are insufficient.
In practice, the desired minimum tensile strength is about 1000 MPa, i.e., about 320 VH (Vickers hardness). The higher the carbon content, the greater the hardness, tensile strength and hardenability but, at the same time, the impact strength and toughness decrease. For this reason, the carbon content is limited to a maximum of 0.18% by weight.
The most important range for the compromise between tensile strength and toughness is 0.09%-0.16% by weight of carbon. However, the ranges 0.06%-0.12% and 0.12%-0.18% are also of interest for applications requiring different core hardnesses.
Silicon provides a major contribution to the tempering strength of this steel and its minimum content is 0.5% by weight. In order to avoid the formation of delta ferrite and to retain sufficient toughness, the silicon content is limited to a maximum of 1.5% by weight. The optimum range is 0.7%-1.3% by weight, but the range 1.3%-1.5% is also of interest.
Chromium contributes to core hardenability and to good tempering strength of the carburized layer, and its minimum content is 0.2% by weight. To avoid embrittlement of the carburized layer by an excess of interlaced carbides, the chromium content must be limited to a maximum of 1.5% by weight. The optimum range is 0.5%-1.2%, but ranges of 0.2%-0.8% and 0.8%-1.5% are also of interest.
The role of molybdenum is identical to that of chromium, and it can keep the elevated temperature hardness high, in particular by forming intragranular carbides in the carburized layer. Its minimum content is 1.1% by weight. However, its embrittling effect on this steel limits its maximum content to 3.5% by weight. The optimum range is 1.5%-2.5%, but ranges of 1.1%-2.3% and 2.3%-3.5% are also of interest.
Vanadium contributes to limiting enlargement of the grain during the carburizing cycles and treatment cycles used. Because of its embrittling effect and its influence on ferrite formation, its content must be limited to a maximum value of 0.4% by weight. The optimum range is 0.15%-0.35%, but ranges of 0.05%-0.25% and 0.25%-0.4% are also of interest.
Manganese, nickel and copper are gamma-forming elements necessary for equilibrating the chemical composition, avoiding ferrite formation and limiting the temperature of the xcex1/xcex3 transformation points. They also provide a major contribution to increasing hardenability, impact strength and toughness but in too high a content, they deteriorate the tempering strength, the elevated temperature hardness and the wear resistance and increase the quantity of residual austenite in the carburized layer.
For these reasons, the manganese content is limited to a maximum of 1.6% by weight. The optimum range is 0.2%-0.7% by weight, but the range 0.7%-1.5% is also of interest. Similarly, the nickel content is limited to the range 1%-3.5% by weight, the optimum range is 2%-3%, but the ranges 1%-2% and 2%-3.5% are also of interest. Finally, copper is limited to a maximum of 2% by weight, the optimum range is 0.3%-1.1% but the range 1.1%-2% can also be of interest.
Cobalt contributes to the tempering strength of the steel and can reduce the AC point. Its effect is substantially the same for low contents. Large quantities of this gamma-forming element stabilizes the residual austenite in the carburized layer. The maximum limit is 4% by weight; contents of less than 1.5% by weight are recommended.
In a second aspect, the invention provides a process for producing carburized and treated parts comprising the following operations:
a. constituting a charge for producing a composition in accordance with the present invention, as described above;
b. melting said charge in an arc furnace;
c. re-heating and thermomechanical transformation of the ingot;
d. homogenizing heat treatment of the structure and refinement of the grain;
e. carburizing; and
f. final heat treatment.
The steel of the invention can be obtained using conventional production techniques but, to obtain the best results as regards impact strength, toughness and fatigue strength, it is recommended that consumable electrode remelting is carried out, either with a slurry (ESR) or under reduced pressure (VAR), following arc furnace melting.
To further enhance this performance, it is also possible to carry out a first melting step by induction under reduced pressure (VIM) and to continue with consumable electrode remelting.
The ingots obtained by one of the above methods undergo re-heating at temperatures of about 1100xc2x0 C. to homogenize the structure, followed by thermomechanical transformations aimed at endowing the product produced from this alloy with a sufficient forging ratio or 3 or more (step c) of the process of the invention). Lower forging ratios can be used, however, for large parts. Conventional processes, such as rolling, forging, drop forging or drawing, are used for these thermomechanical transformations.
A number of implementations can be envisaged regarding step d) of the process of the invention. The transformed products can simply be softened at a temperature below the critical point (AC1), or tempered at a temperature that is above the critical temperature (AC1), assuming a sufficiently slow onset of cooling.
When the best possible characteristics are required, it is preferable, however, to carry out normalization from a temperature above the critical point (AC3), followed by air cooling and softening tempering at a temperature below the critical point (AC1).
By way of indication, the (AC1) critical point temperature is generally in the range from 700xc2x0 C. to 800xc2x0 C., while the (AC3) critical point temperature is generally in the range from 900xc2x0 C. to 980xc2x0 C.
Carburizing, step e) of the process of the invention, can be carried out using conventional means, the carburizing cycle being defined by the skilled person depending on the desired hardening depth, in conventional manner. A low pressure process can in particular be used.
Regarding step f), the final heat treatment of the part, a variety of implementations can be envisaged. It is possible to move directly from the carburizing temperature to the austenitization temperature, then to quench the parts, but it is preferable to allow the parts to cool to ambient temperature after carburizing, then to re-heat to the austenitization temperature, above the critical point (AC3) before quenching them. By way of indication, the austenitization temperature range is 900xc2x0 C.-1050xc2x0 C.
The best characteristics of tensile strength, impact strength, core toughness and superficial hardness of the carburized layer are obtained by carrying out an oil quench after austenitization, but a good compromise between these same characteristics can be achieved by carrying out a gas quench which has the advantage of reducing deformation of the parts during this operation and thus minimizing subsequent machining.
In order to obtain maximum hardness for the carburized layer, and for impact strength and toughness of the sub-layer, it is preferable to temper at the lowest possible temperature compatible with the working temperature. More particularly, a difference of 50xc2x0 C. between the tempering temperature and the working temperature is preferred, the tempering temperature possibly being up to 350xc2x0 C.
When producing this steel in large quantities, a continuous casting technique can be employed to reduce production costs, but a reduction in ductility, impact strength and toughness in particular must be expected.
In a third aspect, the invention provides carburized and treated parts formed from the carburizing steel of the invention which, at ambient temperature, has a core hardness of close to 320 VH to 460 VH, an ISO V impact strength of at least 50 Joules, and more particularly 70 to 150 Joules, a toughness of close to 100 MPam, a superficial carburized layer hardness of close to 750 VH, and which, at 250xc2x0 C., has a superficial carburized layer hardness of close to 650 VH. These parts can advantageously be produced using the production process of the invention, but also using any other process selected as a function of the final application.
The following examples demonstrate that a combination of the elements carbon, manganese, silicon, chromium, nickel, molybdenum, vanadium, copper and cobalt, in the proportions by weight indicated above, results in a steel with, simultaneously, excellent hardness, tensile strength, impact strength, impact strength transition and core toughness characteristics, associated with excellent tempering strength and excellent carburized layer elevated temperature hardness up to working temperatures of 280xc2x0 C.